Device and method for bonding substrates

ABSTRACT

A method for bonding a contact surface of a first substrate to a contact surface of a second substrate comprising of the steps of: positioning the first substrate on a first receiving surface of a first receiving apparatus and positioning the second substrate on a second receiving surface of a second receiving apparatus; establishing contact of the contact surfaces at a bond initiation site; and bonding the first substrate to the second substrate along a bonding wave which is travelling from the bond initiation site to the side edges of the substrates, wherein the first substrate and/or the second substrate is/are deformed for alignment of the contact surfaces.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/341,509, filed Jun. 8, 2021, which is a division of U.S. applicationSer. No. 16/356,325, filed Mar. 18, 2019, now U.S. Pat. No. 11,059,280,which is a division of U.S. application Ser. No. 14/419,664, filed Feb.5, 2015, now U.S. Pat. No. 10,279,575, which is a U.S. National StageApplication of International Application No. PCT/EP2013/061086, filedMay 29, 2013, said patent applications hereby fully incorporated hereinby reference.

FIELD OF THE INVENTION

This invention relates to a method for bonding of a first substrate to asecond substrate.

BACKGROUND OF THE INVENTION

Advancing miniaturization in almost all fields of microelectronics andmicrosystems engineering provides for a continuing development of alltechnologies by which the density of all type of functional units onsubstrates can increase. These functional units include, for example,microcontrollers, memory components, MEMS, all type of sensors ormicrofluid components.

Techniques for increasing the lateral density of these functional unitshave greatly improved in recent years. In some branches ofmicroelectronics or microsystems engineering, the improvement is to apoint that a further increase of the lateral density of the functionalunits is no longer possible. In microchip production, the maximumpossible resolution limit for structures that are producedlithographically has been achieved. In a few years, physical ortechnological limitations will no longer allow any increase in thelateral density of functional units. The industry has been addressingthis problem for some years by the development of 2.5D and 3Dtechnologies. Using these technologies, it is possible to align the sameor even different types of functional units to one another, stack themon top of one another, join them permanently to one another and tonetwork them to one another by corresponding printed circuits.

One of the key technologies for the implementation of these structuresis permanent bonding. Permanent bonding is defined as all methods bywhich substrates can be joined to one another such that they can beseparated only by high energy expenditure and an associated destructionof the substrates. There are different types of permanent bonding.

One of the most important permanent bonding methods is fusion bonding,also called direct bonding or molecular bonding. Fusion bonding isdefined as the process of permanently joining two substrates via theformation of covalent connections. Fusion bonds form mainly on thesurface of nonmetallic-inorganic materials.

Basically, a distinction should be made between a prebond and the actualpermanent bond. A prebond is defined as a connection of surfaces whichforms spontaneously when two surfaces are in contact; their bondingstrength is smaller than the bonding strength of the permanent bond,which is produced by a subsequent heat treatment. The bond strengthcaused by the prebond is. However, sufficient to transport the twosubstrates without causing a shift of the substrates relative to oneanother. Although the bond strength between the two substrates issufficient to easily transport the substrate stack, the bond strength isso low that a repeated, nondestructive separation of the two substratescan take place with special devices. This has the major advantage thatafter a prebond, the structures of the two structures can be measuredand their relative positions, distortions and orientations can bedetermined. If it is established during the measurement process that afaulty orientation and/or a local and/or global distortion of thestructures is present, or there are particles in the interface, thesubstrate stack can be accordingly separated again and reprocessed.After a successful and verified prebond, a permanent bond is produced byheat treatment processes. During the heat treatment process, a chemicaland/or physical strengthening of the connection of the surfaces of thetwo substrates occurs by the supply of thermal energy. This permanentbond is irreversible in the sense that a nondestructive separation ofthe two substrates is no longer possible. Subsequently, it can no longerbe explicitly distinguished between prebond and permanent bond, but ingeneral there is only a bond.

The most common fusion bonds are carried out on silicon and siliconoxide substrates. Silicon is used due to its semiconductor properties asa base material for the production of microelectronic components such asmicrochips and memories. A so-called direct bond can also form betweenhighly polished metal surfaces. The underlying bonding properties differfrom those of a fusion bond, the mechanism with which the two surfacescan make contact with one another by an advancing bonding wave, but canalso be described by the same physics. The joining of two hybridsurfaces by a so-called hybrid bond would also be conceivable. A hybridsurface is defined as a surface consisting of at least two differentmaterials. One of the two materials is generally limited to a smallspace while the second material surrounds the first material. Forexample, metal contacts are surrounded by dielectrics. When a hybridbond is produced by the bonding of two hybrid surfaces, the bonding waveis driven mainly by the fusion bond between the dielectrics, while themetal contacts automatically meet by the bonding wave. Examples ofdielectrics and low-k materials are

-   -   non-silicon based        -   polymers            -   polyimides            -   aromatic polymers            -   parylenes            -   PTFE        -   amorphous carbon    -   silicon based        -   silicate based        -   TEOS (tetraethyl orthosilicate)        -   SiOF        -   SiOCH        -   Glasses (borosilicate glasses, aluminosilicate glasses, lead            silicate glasses, alkali silicate glasses, etc.)    -   general        -   Si₃N₄        -   SiC        -   SiO₂        -   SiCN    -   Silsesquioxanes        -   HSSQ        -   MSSQ

One of the greatest technical problems in permanent joining of twosubstrates is the alignment accuracy of the functional units between theindividual substrates. Although the substrates can be very preciselyaligned to one another by alignment systems, during the bonding processitself, distortions of the substrates can occur. Due to the distortionswhich arise in this way, the functional units will not necessarily becorrectly aligned to one another at all positions. The alignmentaccuracy at a certain point on the substrate can be the result of adistortion, a scaling error, a lens fault (magnification or reductionerror), etc.

In the semiconductor industry, all subtopics which relate to theseproblems are subsumed under the term “overlay.” A correspondingintroduction to this topic can be found, for example, in: Mack, Chris.Fundamental Principles of Optical Lithography—The Science ofMicrofabrication. WILEY, 2007, Reprint 2012.

Each functional unit is designed in a computer before the actualproduction process. For example, printed circuits, microchips, MEMS, orany other structure which can be produced using microsystems technology,are designed in CAD (computer-aided design). During the production ofthe functional units, however, it is shown that there is always adeviation between the ideal functional units, which have been engineeredon a computer, and the real ones, which have been produced in a cleanspace. The differences can be attributed mainly to limitations ofhardware, i.e., engineering problems, but very often to physical limits.Thus, the resolution accuracy of a structure which is produced by aphotolithographic process is limited by the size of the apertures of thephotomask and the wavelength of the light used. Mask distortions aretransferred directly into the resist. Linear motors of machines can onlyapproach reproducible positions within a given tolerance, etc.Therefore, it is no wonder that the functional units of a substratecannot be exactly equal to computer-engineered structures. Allsubstrates, therefore, already have a not negligible deviation from theideal state before the bonding process.

If the positions and/or forms of two opposite functional units of twosubstrates are compared, under the assumption that neither of the twosubstrates is distorted by a joining process, it has been found thatgenerally there is imperfect congruence of the functional units sincethey deviate from the ideal computer model by the above-describedfaults. The most frequent faults are shown in FIG. 8 (copied from:http://commons.wikimedia.org/wiki/File: Overlay—typical model termsDE.svg 24.05.2013 and Mack, Chris. Fundamental Principles of OpticalLithography—The Science of Microfabrication. Chichester: WILEY, p. 312,2007, reprint 2012). According to the figures, it can be roughlydistinguished between global and local and symmetrical and asymmetricaloverlay faults. A global overlay fault is homogenous, therefore,independent of site. It produces the same deviation between two opposingfunctional units regardless of the position. The classic global overlayfaults are faults I and II, which form by a translation or rotation ofthe two substrates to one another. The translation or rotation of thetwo substrates produces a corresponding translational or rotationalfault for all functional units which are opposite at the time on thesubstrates. A local overlay fault arises depending on the location,mainly by problems of elasticity and/or plasticity, in this case causedmainly by the continuously propagating bonding wave. Of the describedoverlay faults, mainly faults III and IV are call run-out faults. Thesefaults arise mainly by a distortion of at least one substrate during abonding process. The functional units of the first substrate withreference to the functional units of the second substrate are alsodistorted by the distortion of at least one substrate. Faults I and IIcan, however, also arise by a bonding process, but are generally sodramatically overlain by faults III and IV that they can only berecognized and measured with difficulty.

In the prior art, there is already a system by which local distortionscan be at least partially reduced. It is a matter of local distortiondue to use of active control elements (see WO2012/083978A1).

In the prior art, there are initial approaches to the correction ofrun-out faults. US20120077329A1 describes a method for obtaining adesired alignment accuracy between functional units of two substratesduring and after bonding by the lower substrate not being fixed. In thisway, the lower substrate is not subjected to boundary conditions and canbond freely to the upper substrate during the bonding process. Animportant feature in the prior art is primarily the flat fixing of asubstrate, generally by means of a vacuum device.

The run-out faults which arise are more dramatic in most cases radiallysymmetrically around the contact site and, therefore, increase from thecontact site to the periphery. In most cases, it is a linearlyincreasing intensification of the run-out faults. Under specialconditions, the run-out faults can also increase nonlinearly.

Under especially optimum conditions, the run-out faults can bedetermined not only by corresponding measuring devices (see EP2463892),but can also be described by mathematical functions. Since the run-outfaults constitute translations and/or rotations and/or scalings betweenwell-defined points, they are preferably described by vector functions.Generally, this vector function is a function f:R²→R², i.e., an imagingstandard which images the two-dimensional definition region of the localcoordinates onto the two-dimensional value range of run-out vectors.Although an exact mathematical analysis of the corresponding vectorfields could not be done, assumptions are made with respect to thefunction properties. The vector functions are with great probabilityc^(n) n>=1 functions, therefore, they are continuously differentiable atleast once. Since the run-out faults increase from the contact-makingpoint toward the edge, the divergence of the vector function willprobably be different from zero. The vector field is therefore withgreat probability a source field.

An advantage of this invention is a device and a method for bonding oftwo substrates with which the bond precision, especially on the edge ofthe substrates, is increased.

This advantage is achieved with the features of the claimed invention.Advantageous developments of the invention are given in the dependentclaims. All combinations of at least two of the features given in thespecification, the claims and/or in the figures fall within the scope ofthe invention. For given values ranges, values which lie within theindicated limits should be considered disclosed as boundary values andable to be claimed in any combination.

SUMMARY OF THE INVENTION

The invention is based on the idea that at least one of the twosubstrates, preferably the second and/or lower substrate, is deformedfor alignment of the contact surfaces solely outside the bond initiationsite before and/or during the bonding, particularly during the travel ofa bonding wave, preferably in fusion bonding. Deformation means a statewhich deviates from an initial state, i.e., the initial geometry of thesubstrates. As claimed in the invention, the bonding is initiated aftercontact-making of the contact surfaces by allowing the first/uppersubstrate to fall or by detaching it. Corresponding bonding means areprovided according to the device.

In the initial state, the substrates on the contact surface aregenerally more or less flat, aside from any structures which projectabove the contact surface (microchips, functional components) andsubstrate tolerance, such as bending and/or thickness fluctuations. Inthe initial state, the substrates have a curvature different from zeroin most cases. For a 300 mm wafer, curvatures of less than 50 μm arecommon. Viewed mathematically, a curvature can be regarded as a measurefor the local deviation of a curve from its planar state. In thisrespect, substrates are examined whose thicknesses are small compared tothe diameter. Therefore, in a good approximation, the curvature of aplane can be addressed. In the case of a plane, the initially mentionedflat state is the tangential plane of the curve at the point at whichthe curvature is being examined. Generally, a body, in the special casethe substrate, does not have a homogeneous curvature so that thecurvature is an explicit function of the site. Thus, it can be, forexample, that a nonplanar substrate in the center has a concavecurvature, at other sites, however, a convex curvature. Subsequently, inthe simplest case, curvatures are only ever described as concave orconvex without going into greater mathematic details, which are known tomathematicians, physicists and engineers.

A basic concept for most embodiments as claimed in the invention iscomprised mainly in that the radii of curvature of two substrates to bebonded to one another are the same at least in the contact-making regionof the substrates, i.e., on one bond front of the bonding wave or on thebond line, or at least deviate only marginally from one another. Thedifference of the two radii of curvature on the bond front/bond line ofthe substrates is therefore smaller than 10 m, preferably smaller than 1m, more preferably smaller than 1 cm, most preferably smaller than 1 mm,still more preferably smaller than 0.01 mm, most preferably of allsmaller than 1 μm. Generally, all embodiments which minimize the radiiof curvature R1 are advantageous. In other words, the invention relatesto a method and a system by which it is possible to bond two substratesto one another such that their local alignment faults, which are calledrun-out faults, become minimum. The invention is furthermore based onthe idea of controlling the two substrates which are to be bonded to oneanother by geometric, thermodynamic and/or mechanical compensationmechanisms such that the factors influencing the forming bonding waveare chosen such that the two substrates do not shift locally to oneanother, and therefore, are correctly aligned. Furthermore, theinvention describes an article comprised of two substrates which havebeen bonded to one another with a run-out fault which has been reducedas claimed in the invention.

One process which is characteristic in bonding, especially permanentbonding, preferably fusion bonding, is the point contact-making of thetwo substrates which is as centric as possible. Generally, thecontact-making of the two substrates can also take placenon-centrically. The bonding wave propagating from a non-centric contactpoint would reach different locations of the substrate edge at differenttimes. The complete mathematical-physical description of bonding wavebehavior and the resulting run-out fault compensation would becorrespondingly complicated. But generally, the contact-making pointwill not be far from the center of the substrate so that the possiblyresulting effects are negligible at least on the edge. The distancebetween a possible non-centric contact-making point and the center ofthe substrate is smaller than 100 mm, preferably smaller than 10 mm,more preferably smaller than 1 mm, most preferably smaller than 0.1 mm,most preferably of all smaller than 0.01 mm. In the followingdescription, contact-making generally means centric contact-making. In awider sense, the center is preferably defined as the geometrical centerpoint of an underlying ideal body, compensated by asymmetries, ifnecessary. In commercial wafers with a notch, the center is thereforethe midpoint of a circle, which surrounds the ideal wafer without anotch. In commercial wafers with a flat (flattened side), the center isthe midpoint of the circle which surrounds the ideal wafer without aflat. Analogous considerations apply to substrates of any shape. Inspecial embodiments, it can useful to define the center of gravity ofthe substrate as the center. In order to ensure exact, centric, pointcontact-making, an upper mounting apparatus (substrate holder) having acentric hole and a pin which can be moved by translation in the hole isprovided with a radially symmetrical fixing. The use of a nozzle, whichuses a fluid, such as a gas instead of the pin to apply pressure, wouldalso be conceivable. Furthermore, the use of these components can evenbe completely abandoned when devices are provided which can move the twosubstrates toward one another by a translation movement, under theassumption that at least one of the two substrates, preferably the uppersubstrate, has an impressed curvature due to gravitation in thedirection of the other substrate and therefore, in the mentionedtranslational approach to one another, at a relatively small distance tothe corresponding second substrate, automatically makes contact.

The radially symmetrical fixing is either attached vacuum holes, a roundvacuum lip, or comparable vacuum elements, by which the upper wafer canbe fixed. The use of an electrostatic mounting apparatus is alsoconceivable. The pin in the centric hole of the upper substrate holderis used for controllable deflection of the fixed upper substrate.

After completed contact-making of the centers of the two substrates, thefixing of the upper substrate holder is released. The upper substratedrops on the one hand due to gravity and, on the other hand, due to abond force acting along the bonding wave and between the substrates. Theupper substrate is connected radially to the lower substrate from thecenter to the side edge. Thus, formation of a radially symmetricalbonding wave, which runs from the center to the side edge, occurs.During the bonding process, the two substrates press the gas presentbetween the substrates, i.e., air, in front of the bonding wave, andthus provide for a bond boundary surface without gas inclusions. Theupper substrate lies essentially on a type of gas cushion whiledropping.

The first/upper substrate, after initiation of the bond at one bondinitiation site, is not subject to additional fixing, therefore asidefrom the fixing at the bond initiation site, the first/upper substratecan move freely and can also be distorted. Each circle segment, which isinfinitesimally small with respect to its radial thickness, will besubjected to a distortion by the bonding wave which is advancing thestress states which occur on the bonding wave front, and the existinggeometrical boundary conditions. But since the substrates representrigid bodies, the distortions add up as a function of the distance fromthe center. This leads to run-out faults which are to be eliminated bythe method and the device as claimed in the invention.

Thus, the invention also relates to a method and a device for reducing,or even entirely avoiding, the run-out fault between two bondedsubstrates, especially by thermodynamic and/or mechanical compensationmechanisms during bonding. Furthermore, the invention treats acorresponding article which is produced with the device and the methodas claimed in the invention.

In a first embodiment of the invention, the first, i.e., the lower,receiving or mounting apparatus on the receiving or mounting surface forreceiving the first substrate is ground and/or polished and/or lappedconvexly or concavely. Preferably, the mounting apparatus is convexlyground so that a substrate fixed on it is curved in the direction of thecontact-making point or the bond initiation site.

A radius of curvature of the first and/or second receiving surfaces isespecially greater than 0.01 m, preferably greater than 0.1 m, morepreferably greater than 1 m, still more preferably greater than 10 m,most preferably greater than 100 m, most preferably of all greater than1000 m. In one special embodiment, the radius of curvature of thefirst/lower receiving apparatus is the same size, especially within thesame order of magnitude of one power of ten, as the radius of curvatureof the second/upper substrate produced by the second/upper receiving ormounting apparatus by actuating means, especially a pin. This results inan initial position which is symmetrical with respect to the geometryfor bonding.

It is advantageous if the first/lower receiving surface is ground suchthat the physical asymmetry is corrected by the gravitational fieldacting normally on the mounting surface such that the bonding wave frontis always moving within the same horizontal plane.

The first and/or second substrate is preferably radially symmetrical.Although the substrate can have any diameter, the wafer diameter isespecially 1 inch, 2 inches, 3 inches, 4 inches, 5 inches, 6 inches, 8inches, 12 inches, 18 inches or larger than 18 inches. The thickness ofthe first and/or second substrate is preferably between 1 μm and 2000μm, preferably between 10 μm and 1500 μm, more preferably between 100 μmand 1000 μm.

In special embodiments, a substrate can have a rectangular shape or atleast one which deviates from a round shape. Hereinafter, a substrate isdefined as a wafer.

In a second embodiment of the lower/first mounting apparatus, the radiusof curvature of the lower/first mounting surface is adjustable. In onesimple embodiment, the lower mounting apparatus has a support platewhich is thick enough to not be deformed by unwanted externalinfluences, but which has been produced to be thin enough to be broughtinto a convex or concave shape by a controlled force which is actingfrom underneath. In particular, the support plate has a bendingstiffness of greater than 10⁻⁷ Nm², preferably greater than 10⁻³ Nm²,more preferably greater than 1 Nm², still more preferably greater than10³ Nm², most preferably of all greater than 10⁷ Nm². According to oneembodiment of the invention, an interior part of the lower/firstmounting apparatus is comprised of a highly elastic membrane which canbe expanded and/or contracted pneumatically and/or hydraulically and/orpiezoelectrically. The pneumatic and/or hydraulic and/or piezoelectricelements are preferably uniformly distributed and can be triggeredindividually.

In another embodiment, the lower/first mounting apparatus is made suchthat the lower/first substrate is deformed, i.e., laterally compressedor expanded, in a controlled manner by heating and/or cooling means evenbefore contact-making, by the amount which is necessary in latercontact-making to best, ideally completely, compensate for the run-outfault which arises. Since the fixing of the lower/first substrate inthis embodiment only takes place after corresponding deformation, aspecial value need not be imposed on the coefficient of thermalexpansion of the lower/first substrate or the lower/first mountingapparatus.

In another special procedure, the contact-making of the lower/firstsubstrate takes place prior to a heating and/or cooling process. Byfixing prior to the heating and/or cooling process, the lower/firstsubstrate follows the thermal expansion of the lower/first mountingapparatus so that its coefficient of thermal expansion can be used todictate the (thermal) expansion of the substrate. Preferably, thecoefficients of thermal expansion of the lower/first substrate and thelower/first mounting apparatus are the same so that in the heatingand/or cooling process no thermal stresses, or at least small thermalstresses, arise in the lower/first substrate. It would also beconceivable for the coefficients of thermal expansion to be different.In the case of different coefficients of thermal expansion, thelower/first substrate in the middle will follow the thermal expansion ofthe lower/first mounting apparatus.

The temperature difference to be set between the first and secondmounting apparatus are less than 20° C., preferably less than 10° C.,more preferably less than 5° C., most preferably less than 2° C., mostpreferably of all less than 1° C. Each mounting apparatus is heatedthrough as homogeneously as possible. In particular, there is atemperature field whose temperature difference at any two points issmaller than 5° C., preferably smaller than 3° C., more preferablysmaller than 1° C., most preferably smaller than 0.1° C., mostpreferably of all smaller than 0.05° C.

In another embodiment, the first mounting apparatus is designed suchthat the mounting apparatus on the mounting surface can be deformed,compressed and/or expanded, in a controlled manner by mechanicalactuating means. The first substrate, which is fixed on the surface ofthe first mounting apparatus, is deformed due to its small thicknesswith reference to the mounting apparatus by the deformation of themounting apparatus. The mounting apparatus is deformed with pneumaticand/or hydraulic and/or piezoelectric actuators which have been arrangeddistributed around the substrate mount, preferably radially andsymmetrically. For a completely symmetrical, purely radial distortion,at least three actuators are needed which are arranged at an angulardistance of 120°. Preferably, more than 5 actuators, more preferablymore than 10 actuators, preferably more than 20 actuators, stillpreferably more than 30 actuators, most preferably of all more than 50actuators are used.

In another embodiment, the contact surfaces of the two substrates arebonded to one another in a vertical position. The task of thisembodiment is comprised mainly in reducing the deformation of the wafersby gravitation, i.e., preferably arranging them at least symmetrically,and entirely preventing and/or compensating for a deformation bygravitation. Preferably, in the vertical position, the two substratesare curved at the same time symmetrically to the bond initiation site byone actuating means each, in particular one pin each, toward the bondinitiation site so that the convex surfaces can make contact in the bondinitiation site. The automatic bonding process with a bonding wave isstarted by detaching at least one of the substrates from the mountingsurface.

The embodiments as claimed in the invention are preferably operated in adefined, especially controllable atmosphere, especially under normalpressure.

All mentioned embodiments can be implemented in one special version in alow vacuum, more preferably in a high vacuum, still more preferably inan ultrahigh vacuum, in particular with a pressure of less than 100mbar, preferably less than 10⁻¹ bar, more preferably less than 10⁻³ bar,still more preferably less than 10⁻⁵ bar, most preferably less than 10⁻⁸bar.

In a further embodiment, at least one factor influencing thepropagation, in particular the propagation velocity, of the bonding waveand/or at least one factor influencing the alignment of the contactsurfaces, is controlled. The bonding wave is monitored with respect toits velocity. The velocity is controlled indirectly via the compositionand/or the density and/or the temperature of a gas in the atmosphere inwhich bonding is done. Although the method is preferably to be carriedout in a low pressure atmosphere, preferably in a vacuum, it can beadvantageous to carry out the bonding process in another atmosphere, inparticular in the range of the normal pressure. Due to the pointcontact, the bonding wave in the bonding always travels radiallysymmetrically from the center to the side edge, and in this processpresses an annular gas cushion in front of it. Along the roughlycircular ring-shaped bond line (bond front) of the bonding wave, such alarge bond force prevails that inclusions of gas bubbles cannot arise atall. The upper/second substrate therefore lies on a type of gas cushionduring the bonding process.

It is established by the selection of a gas/gas mixture and definitionof the properties of the gas cushion how fast and how strongly thesecond substrate can be lowered and/or expanded. Furthermore, thevelocity of the bonding wave can also be controlled via the propertiesof the gas.

The composition of the gas mixture is chosen according to another,independent aspect of the invention. Preferably, gases with types ofatoms and/or molecules that are as light as possible and which have acorrespondingly low inertia at a given temperature are used. Therefore,the molar mass at least of one of the gas components is less than 1000g/mole, preferably less than 100 g/mole, more preferably less than 10g/mole, most preferably of all less than 1 g/mole. More preferably, thedensity of the gas mixture used is set to be in particular as small aspossible, and/or the temperature is set to be in particular as high asnecessary. The gas density is set to be smaller than 10 kg/m³,preferably smaller than 1 kg/m³, more preferably smaller than 0.1 kg/m³,most preferably smaller than 0.01 kg/m³. The temperature of the gas isset to be greater than 0° C., preferably greater than 100° C., morepreferably greater than 200° C., most preferably greater than 300° C.,most preferably of all greater than 400° C. The aforementionedparameters are chosen such that the selected gas mixture or individualcomponents of the gas mixture do not condense. In this way liquidinclusions on the surface of the substrates during the bonding processare avoided.

Analogous considerations apply to gas mixtures whose thermodynamicproperties are shown in multicomponent phase diagrams. By changing thecomposition and/or pressure and/or temperature of the gas mixture, thekinematics of the first and/or second substrate is influenced and thusthe run-out fault is also reduced.

Preferably, all variable parameters are chosen such that the bondingwave propagates with a velocity as optimum as possible with respect tothe existing initial and boundary conditions. Mainly for an existingatmosphere, at normal pressure, a velocity of the bonding wave as slowas possible is advantageous. The velocity of the bonding wave is lessthan 200 cm/s, more preferably less than 100 cm/s, more preferably lessthan 50 cm/s, most preferably less than 10 cm/s, most preferably of allless than 1 cm/s. In particular, the velocity of the bonding wave isgreater than 0.1 cm/s. In particular, the velocity of the bonding wavealong the bond front is constant. In a vacuum environment, the velocityof the bonding wave automatically becomes faster since the substrateswhich are being joined along the bond line need not overcome anyresistance by a gas.

According to another independent aspect of the invention, a stiffeningplate is inserted between the mounting surface and the upper/secondsubstrate on the upper/second mounting apparatus. The stiffening plateis temporarily bonded to the substrate and changes the behavior of theupper/second substrate during bonding. The connection between thestiffening plate and the upper/second substrate takes place by aconstruction-engineering fixing in the stiffening plate. The fixing ispreferably vacuum fixing. Electrostatic fixings, very thin mechanicalfixings which rechuck the substrates on the edge, and adhesion fixingsby a highly polished stiffening plate surface would also be conceivable.

According to another independent aspect of the invention, the run-outfault is set by a very small distance between the two substrates priorto contact-making and/or outside the bond initiation site. The distanceis in particular less than 100 μm, preferably less than 50 μm, morepreferably less than 20 μm, most preferably less than 10 μm.

It is preferable that the radius of curvature of the two substrates, onthe bond front, deviates by less than 5% from one another, morepreferably is the same.

In another special development of all embodiments, the lower/firstmounting apparatus and/or the upper/second mounting apparatus have asactuating means, such as centric holes and a pin, by which a convexcurvature of the respective substrates can be effected in the directionof the bond initiation site.

The above described steps and/or movements and/or sequences, especiallyof the pins for deflection of the substrates, the approach of thesubstrates to one another, monitoring of the temperature, of thepressure and of the gas composition, are controlled preferably via acentral control unit, i.e., a computer with control software.

The substrates can be received and fixed on a receiving or mountingapparatus in any conceivable manner with any known technology. Vacuumsample holders, electrostatic sample holders, sample holders withmechanical clamping, are in particular conceivable according to theinvention. Preferably, the substrates are fixed solely on a circlesegment which is as far as possible outside in the region of the sideedge in order to afford the substrates maximum flexibility and freedomto expand within the fixing.

Another independent aspect of the invention comprises contact-making ina manner as coordinated as possible and at the same timequasi-independently by at least one of the substrates being exposed to aprestress. This prestress runs concentrically to the middle M of onecontact surface of the substrate radially to the outside beforecontact-making. Then, only the start of contact-making is influenced.After contact-making of a section, in particular the middle M of thesubstrate, the substrate is released and bonds to the opposite substratecontrolled automatically based on its prestress. The prestress isachieved by a deformation of the first substrate by deformation means.The deformation means, based on their shape, acting on the side facingaway from the bond side and the deformation, is controllable by usingdifferent (especially interchangeable) deformation means. The controlalso takes place by the pressure or the force with which the deformationmeans act on the substrate. In this respect, it is advantageous toreduce the effective mounting surface of the mounting apparatus with thesemiconductor substrate so that the semiconductor substrate is onlypartially supported by the mounting apparatus. In this way, the smallercontact surface yields smaller adhesion between the wafer and the sampleholder or the mounting apparatus. Fixing is applied solely in the regionof the periphery of the semiconductor substrate (first substrate) sothat there is efficient fixing with an effective mounting surfacesimultaneously as small as possible between the mounting contour of themounting apparatus and the semiconductor substrate. Thus, at the sametime, gentle and reliable detachment of the semiconductor substrate ispossible since the detachment forces necessary for detaching the waferare as small as possible. The detachment is mainly controllable, byreducing the negative pressure on the mounting surface. “Controllable”means that after the contact of the wafer with a second wafer, the waferon the sample holder remains still fixed and only by dedicated(controlled) reduction of the negative pressure on the mounting surfaceis a detachment of the substrate (wafer) from the sample holder(mounting apparatus) effected, from the inside to the outside. Theembodiment as claimed in the invention leads mainly to the detachmentbeing able to be effected by very small forces.

The substrates (wafers) are aligned to one another prior to the bondingprocess in order to ensure identical congruence (exact alignment,especially with a precision of less than 2 μm, preferably less than 250nm, more preferably less than 150 nm, most preferably less than 100 nm)of corresponding structures on their surfaces. In the bond methodaccording to the invention, the wafers are not placed flat on oneanother, but are brought into contact with one another first in themiddle M by one of the two wafers being pressed lightly against thesecond wafer or being deformed accordingly in the direction of theopposite wafer. After detachment of the deformed (in the direction ofthe opposite wafer) deflected wafer, continuous and more uniform weldingtakes place which is at least largely automatic and which is associatedwith the minimum force and with the minimum mainly horizontaldistortions along the bond front by the advance of a bonding wave.

Another independent aspect of the invention comprises controlling thedeformation of the first substrate and/or of the second substratedepending on given factors influencing the travel of the bonding wave.The influencing factors include among others the ambient pressure of theatmosphere surrounding the substrates, the type of gas/gas mixturepresent in the atmosphere, the temperature, the distance between thesubstrates outside the bond initiation site, the bond strength of thesubstrates, any pretreatment steps, the composition of the surfaces, thesurface roughness, the materials on the surface and the waferthickness/bending strength.

To the extent the bond initiation site is located in the center of thecontact surfaces of the substrates, uniform, concentric travel of thebonding wave can be accomplished.

It is advantageous if the deformation of the first substrate and/or ofthe second substrate takes place in the lateral direction and/orconvexly and/or concavely, still more preferably mirror-symmetrically.In other words, the deformation takes place by expansion or compressionor curving of the first substrate and/or of the second substrate.

Preferably, the substrates have roughly identical diameters D1, D2 whichdeviate from one another in particular by less than 5 mm, preferablyless than 3 mm, still more preferably less than 1 mm.

According to another aspect of the invention, the deformation takesplace by mechanical actuating means and/or by temperature control of thefirst and/or second mounting apparatus.

By the first substrate and/or the second substrate being fixed solely inthe region of the side walls on the first and/or second mountingsurfaces, the deformation as claimed in the invention can beaccomplished more easily.

Features disclosed according to the device should also be considered asdisclosed according to the method and vice versa.

Other advantages, features and details of the invention will becomeapparent from the following description of preferred exemplaryembodiments using the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross section of a first embodiment of thedevice as claimed in the invention,

FIG. 2 shows a schematic cross section of a second embodiment of thedevice as claimed in the invention,

FIG. 3 shows a schematic cross section of a third embodiment of thedevice as claimed in the invention,

FIG. 4 shows a schematic cross section of a fourth embodiment of thedevice as claimed in the invention,

FIG. 5 shows a schematic cross section of a fifth embodiment of thedevice as claimed in the invention,

FIG. 6 shows a schematic of the method step of bonding as claimed in theinvention,

FIG. 7 a shows a schematic of a bonded substrate pair with an alignmentfault dx in the region of one side edge of the substrates,

FIG. 7 b shows a schematic enlargement of two substrates in the regionof a bonding wave as claimed in the invention,

FIG. 7 c shows a schematic enlargement of two substrates withoutalignment faults/run-out faults,

FIG. 7 d shows a schematic enlargement of two substrates with alignmentfaults/run-out faults and

FIG. 8 shows a symbolic representation of the possible overlay orrun-out fault.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the figures, the same components and components with the samefunction are labeled with the same reference numbers.

FIG. 1 shows a lower first mounting apparatus 1 as a substrate sampleholder, which apparatus 1 is comprised of a base body 9 and a mountingbody 2. The mounting body 2 has a first mounting surface 2 o which islocated to the top in the direction of an opposing second mountingapparatus 4. In the embodiment illustrated, first mounting surface 2 ois convexly curved. The mounting body 2 is made interchangeable as amodule and can be separated from the base body 9. The base body 9 isthus used as an adapter between the mounting body 2 and a lower mountingunit of the bonder (not shown). This makes it possible to carry out aprompt change between different modular mounting bodies 2 with differentradii R of curvature if necessary.

On the mounting body 2, there are fixing means 6 in the form of vacuumpaths with which a lower first substrate 3 can be fixed on the mountingsurface 2 o.

Since the radius of curvature R is preferably very large, and thus thecurvature is essentially undetectable with the naked eye (therepresentation in the figures is highly exaggerated and schematic), itis to execute the first mounting apparatus without fixing means 6 and tofinally place the first substrate on the mounting surface 2 o. Adhesionby electrostatic, electrical or magnetic fixing means is alsoconceivable as claimed in the invention.

The second mounting apparatus 4, which is made as a substrate sampleholder, is comprised of a base body 2′ with a second mounting surface 2o′ which can be aligned in particular in concentric sectionsequidistantly to corresponding concentric sections of the first mountingsurface 2 o.

The base body 2′ has an opening 5 in the form of a hole and fixing means6′ similarly to the fixing means of the first mounting apparatus 1.

The fixing means 6′ are used to fix an upper, second substrate 8 on oneside facing away from the contact surface 8 k of the second substrate 8.

Actuating means in the form of a pin 7 are used for deformation (here:deflection) and thus for the primarily point approach of the secondsubstrate 8 to the curved first substrate 3, especially in the region ofa curvature maximum.

In one special embodiment, it is conceivable to make the mounting body 2of temperature-resistant and/or corrosion-resistant material to form astretchable component which can be expanded and contracted pneumaticallyand/or hydraulically, in particular a pillow.

FIG. 2 shows a second embodiment with a mounting body 2″ in which thefirst mounting surface 2 o can be deformed in a controlled manner by anactuating element 10, which in the embodiment shown is a pull rod and/orpush rod. The mount 2″ has a concentrically running fixing section 16and a deformation section 17 which encompasses the first mountingsurface 2 o. The deformation section 17 has at least predominantly aconstant thickness and thus a predominantly constant bending stiffness.The actuating element 10 is located and fixed, on the actuating side ofthe deformation section 17 facing away from the mounting surface 2 o. Bymeans of the actuating element 10, the deformation section 17 can bedeformed in the micrometer range and can be curved convexly andconcavely. The actuating element 10 travels more than 0.01 μm,preferably more than +/−1 μm, more preferably more than +/−1 μm, stillmore preferably more than +/−100 μm, most preferably more than +/−1 mm,most preferably of all more than +/−10 mm. Otherwise, the embodimentaccording to FIG. 2 corresponds to the embodiment according to FIG. 1 .

FIG. 3 shows a third embodiment with a first mounting apparatus 1 with amounting body 2′″. The mounting body 2′″ has the first mounting surface2 o for accommodating the first substrate 3. Furthermore, the firstmounting apparatus 1 in this embodiment has temperature control means 11for temperature control (heating and/or cooling) of the mounting body2′″ at least in the region of the first mounting surface 2 o, andpreferably the entire mounting body 2′″.

In the first procedure, the first substrate 3 is fixed on the heatedmounting body 2′″ only after reaching its expansion which has beencaused by the temperature difference. In this way, the first substrate 3expands according to its own coefficient of thermal expansion.

In a second procedure, the first substrate 3 is fixed on the mountingbody 2′″ before thermal loading by the temperature control means 11. Bychanging the temperature control means 11, the mounting body 2′″ andthus the first mounting surface 2 o with the first substrate 3 fixed onit expand in the lateral direction. Preferably, the mounting body 2″ hasessentially the same coefficient of thermal expansion as the firstsubstrate 3. Otherwise, the third embodiment corresponds to theabove-described first and second embodiments.

FIG. 4 shows a fourth embodiment with a base body 9′ with a mountingsection 18 for accommodating a mounting body 2 ^(IV). The base body 9′comprises a shoulder section 19 which adjoins the mounting section 18and which is made circumferential. The shoulder section 19 is used as astop for actuating elements 12, which are used for deformation of themounting body 2 ^(IV) in the lateral direction. The actuating elements12 in the form of plurality of pulling and/or pushing elements 12 arearranged and distributed on the lateral periphery of the mounting body 2^(IV). The actuating elements 12 are used for deformation of themounting body 2 ^(IV) in the lateral direction, by mechanical expansionand/or compression, preferably in the micrometer range. The mountingbody 2 ^(IV) is expanded/compressed by more than 0.01 μm, preferablymore than +/−1 μm, more preferably more than +/−1 μm, still morepreferably more than +/−100 μm, most preferably more than +/−1 mm, mostpreferably of all more than +/−10 mm. The actuating elements 12 can bemade as purely mechanical and/or pneumatic and/or hydraulic and/orpiezoelectric components.

Otherwise, the fourth embodiment corresponds to the above describedfirst, second and third embodiments. In the fourth embodiment it isespecially important that the adhesion between the substrate 1 and themounting body 2 ^(IV) is so great that the substrate 1 during elongationor compression of the mounting body 2 ^(IV) is likewise accordinglyelongated or compressed by the actuating elements 12.

FIG. 5 shows a fifth embodiment in which the first mounting apparatus 1and the second mounting apparatus 4 are vertically aligned. The firstmounting apparatus 1 has a base body 9″ and a mounting body 2 ^(V) whichis fixed by the base body 9″. The mounting body 2 ^(V) encompasses thefirst mounting surface 2 o which is located vertically in thisembodiment and on which the first substrate 3 is fixed via fixingelements 6.

The second mounting apparatus 4 encompasses a base body 9′″ which islocated oppositely for accommodating and fixing a mounting body 2 ^(VI).The mounting body 2 ^(VI) is used to accommodate and fix the secondsubstrate 8 on its vertically arranged mounting surface 2 o′. Fordeformation of the second substrate 8, an opening 5 (analogous toopening 5 according to FIG. 1 ) with an actuating means in the form of apin 7 is provided. Pin 7 is made to deform the second substrate 8through the opening 5, specifically on one side facing away from thecontact surface 8 k of the second substrate 8. The pin 7 defines a bondinitiation site 20 when the substrates 3, 8 make contact by deformationof the second substrate 8.

Aside from the opening 5 and the pin 7, the first mounting apparatus 1and the second mounting apparatus 4 are made symmetrical in theembodiment according to FIG. 5 . Preferably, the first mountingapparatus also has a corresponding opening and a corresponding pin sothat a symmetrical deformation of the substrates 3, 8 is enabled. Bysimultaneously releasing the fixing means 6, 6′ after the deformedsubstrates 3, 8 make contact at a bond initiation site 20 which islocated in particular in the center of the substrates 3, 8, thesubstrates 3, 8 behave identically so that even for the lack of aninfluence of a gravitational force on the deformation in the directionof the contact surfaces 3 k, 8 k onto one another no alignment faultsoccur during the advance of the bonding wave. This applies when thepredominant number of factors, which influence the bonding wave oralignment faults, i.e., the thickness of the substrates or the bendingstiffness of the substrates, is the same. The bending stiffness is theresistance which a body opposes to bending impressed on it. The higherthe bending stiffness, the greater the bending moment must be to achievethe same curvature.

The bending stiffness is a pure material and geometrical property whichis independent of the bending moment (assuming that the bending momentdoes not change the moment of inertia by a change in the cross section).The cross section of a wafer through its center is a very goodapproximation of a rectangle with a thickness t3 and wafer diameter D.The bending stiffness is the product of the modulus of elasticity andthe planar moment of inertia for homogeneous cross sections. The planarmoment of inertia of a rectangle around an axis normal to the thicknesswould be directly proportional to the third power of the thickness.Therefore, the moment of inertia and thus the bending moment areinfluenced by the thickness. The bending moment arises by the action ofthe gravitational force along the substrate. For a constant bendingmoment, e.g., a constant gravitational force, substrates with greaterthicknesses due to their greater bending moments are less curved thansubstrates with lower thicknesses.

FIG. 6 schematically shows the bonding process, wherein a radius R ofcurvature of the first mounting apparatus 1 is shown highly exaggeratedfor illustration purposes. The radius of curvature is several meters atdiameters of the substrates 3, 8 in the range from 1 inch to 18 inchesand thicknesses of the substrates 3, 8 in the range from 1 μm to 2000μm.

After contact-making of the substrates 3, 8 on contact surfaces 3 k, 8 kin the region of the bond initiation site 20, which site 20 lies in thecenter of the substrates 3, 8, and after cancellation of the fixing(release) of the second substrate 8 from the second mounting apparatus4, bonding begins. A bonding wave with a bond front 13 runsconcentrically from the bond initiation site 20 to the side edges 8 s, 3s of the substrates 3, 8.

In doing so, the bonding wave displaces the gas 15 (or gas mixture 15)which is shown by arrows between the contact surfaces 3 k, 8 k.

The substrate 3 is deformed by the mounting apparatus 1 such that thealignment faults of corresponding structures 14 of the substrates 3, 8are minimized.

The substrate 8 deforms during travel of the bonding wave (after bondinitiation and release from the mounting apparatus 4) based on theacting forces: gas pressure, gas density, velocity of the bonding wave,weight, natural frequency (spring behavior) of the substrate 8.

In the illustrated exemplary embodiment which corresponds to the firsttwo embodiments according to FIGS. 1 and 2 , the deformation takes placeby curvature of the substrates 3, 8, a radius R of curvature of theupper substrate 8, especially at each instant on the bond front 13,corresponding essentially to the radius R1 of curvature of the lowersubstrate 3. If one of the two mounting surfaces 2 o, 2 o′ is flat andthus also the radius of the corresponding substrate 3 or 8 which issupported on the flat mounting surface 2 o or 2 o′ is infinitely large,the radius of the correspondingly opposite substrate 8 or 3 is set to becorrespondingly large, in the boundary case infinitely large. Thus,inventive compensation of the run-out fault by two substrates 3 and 8approaching one another, whose radii of curvature are infinitely large,therefore whose contact surfaces 3 k, 8 k are parallel to one another,is also disclosed. This inventive special case would be suitable mainlyin a vacuum environment in order to join the two substrates 3 and 8 toone another since it would not be necessary to bond the two substrates 3and 8 to one another by a bonding wave which is pushing an amount of gasin front of it out of the bond interface and which is propagating fromthe bond center. The difference of the radii R1 and R of curvature isespecially smaller than 100 m, preferably smaller than 10 m, morepreferably smaller than 1 m, most preferably smaller than 1 cm, mostpreferably of all smaller than 1 mm.

It is conceivable to control the atmosphere by choosing the gas 15 orthe gas mixture 15 and the pressure and/or the temperature the bondvelocity.

FIGS. 7 a to 7 d illustrate in enlarged form possible alignment faultsdx according to FIGS. 7 a and 7 d , which as claimed in the inventionaccording to FIGS. 7 b and 7 c are at least predominantly eliminated bythe deformation of the substrates 3, 8.

The described method steps, especially movements and parameters, arecontrolled by an especially software-supported control apparatus (notshown).

REFERENCE NUMBER LIST

-   -   1 first receiving/mounting apparatus    -   2, 2′, 2″, 2′″, 2 ^(IV), 2 ^(V), 2 ^(V1) receiving/mounting body    -   2 o first receiving/mounting surface    -   2 o′ second receiving/mounting surface    -   3 first substrate    -   3 k first contact surface    -   3 s side edge    -   4 second receiving/mounting apparatus    -   5 opening    -   6, 6′ fixing means    -   7 pin    -   8 second substrate    -   8 k second contact surface    -   8 s side edge    -   9, 9′, 9″, 9′″ base body    -   10 actuating element    -   11 temperature control means    -   12 actuating element    -   13 bond front    -   14, 14′ structure    -   15 gas/gas mixture    -   16 fixing section    -   17 deformation section    -   18 mounting section    -   19 shoulder section    -   20 bond initiation site    -   dx alignment fault    -   d1, d2 diameter    -   R, R1 radius of curvature

1. A method for bonding a first substrate to a second substrate onrespective contact surfaces of the substrates, the method comprising:accommodating the first substrate on a first mounting surface of a firstmounting apparatus and the second substrate on a second mounting surfaceof a second mounting apparatus; bringing the contact surface of thefirst substrate into contact with the contact surface of the secondsubstrate at a bond initiation site; and bonding the first substrate tothe second substrate along a bonding wave which is traveling from thebond initiation site to side edges of the substrates, the bondingcomprising adjusting the first mounting surface and/or the secondmounting surface in such a way that physical asymmetry of the firstmounting surface and/or the second mounting surface is corrected by agravitational field acting normally on the first mounting surface and/orthe second mounting surface to enable a front of the bonding wave tomove within a same horizontal plane and enable at least partialcompensation of deformation of the first substrate and/or the secondsubstrate.
 2. The method as claimed in claim 1, wherein the deformationof the first substrate and/or the second substrate depends on givenfactors that influence the traveling of the bonding wave.
 3. The methodas claimed in claim 1, wherein the bond initiation site is located in acenter of the contact surfaces.
 4. The method as claimed in claim 1,wherein the deformation of the first substrate and/or the secondsubstrate takes place in a lateral direction, convexly, concavely, or acombination one or more thereof.
 5. The method as claimed in claim 1,wherein the deformation of the first substrate and/or the secondsubstrate comprises one of expansion, compression, or curving the firstsubstrate and/or the second substrate.
 6. The method as claimed in claim1, wherein diameters of the substrates deviate from one another by lessthan 5 mm.
 7. The method as claimed in claim 1, wherein the deformationof the first substrate and/or the second substrate takes place throughmechanically actuating and/or controlling a temperature of the firstmounting apparatus and/or the second mounting apparatus.
 8. The methodas claimed in claim 1, wherein the first substrate and/or the secondsubstrate are fixed solely in a region of the side edges on the firstand/or second mounting surfaces.
 9. A device for bonding of a firstsubstrate to a second substrate on respective contact surfaces of thesubstrates, the device comprising: a first mounting apparatus configuredto accommodate the first substrate on a first mounting surface; a secondmounting apparatus configured to accommodate the second substrate on asecond mounting surface; contact means for bringing the contact surfaceof the first substrate into contact with the contact surface of thesecond substrate at a bond initiation site; and means for bonding thefirst substrate to the second substrate along a bonding wave which istraveling from the bond initiation site to side edges of the substrates,the bonding means comprising means for adjusting the first mountingsurface and/or the second mounting surface in such a way that physicalasymmetry of the first mounting surface and/or the second mountingsurface is corrected by a gravitational field acting normally on thefirst mounting surface and/or the second mounting surface to enable afront of the bonding wave to move within a same horizontal plane andenable at least partial compensation of deformation of the firstsubstrate and/or the second substrate.
 10. The device as claimed inclaim 9, further comprising: deformation means for deforming the firstsubstrate and/or the second substrate for alignment of the contactsurfaces outside the bond initiation site before and/or during thebonding,
 11. The device as claimed in claim 10, wherein the deformationmeans is configured to encompass the first mounting apparatus, andwherein the first mounting apparatus is deformable on the first mountingsurface in a lateral direction, convexly, concavely, or in a combinationof one or more thereof.
 12. The device as claimed in claim 10, whereinthe deformation means is configured to encompass the second mountingapparatus, and wherein the second mounting apparatus is deformable onthe second mounting surface in a lateral direction, convexly, concavely,or in a combination of one or more thereof.
 13. The device as claimed inclaim 10, wherein the deformation means comprises one or more ofmechanical actuating means for mechanically actuating the first and/orsecond mounting apparatus and temperature control means for controllinga temperature of the first and/or second mounting apparatus.
 14. Thedevice as claimed in claim 9, further comprising: fixing means forfixing the first substrate and/or the second substrate solely in aregion of the side edges on the first and/or second mounting surfaces.